Menu:

Version:

Sept 1, 2016:
Revised: v1.5

ZL2PD Builds the Lydford 40m QRP SSB Transceiver

The
Lydford transceiver was a HF QRP SSB transceiver kit produced by Walford
Electronics in the UK during 2013-15. The kit instructions allowed it to be built for
use on any single band from 80m to 20m. I built my version for use on
40m during 2014. On this webpage, I describe
my construction of the kit and some of the minor modifications and accessories I made for
it.

PLEASE NOTE: THIS KIT IS NO LONGER AVAILABLE

Introduction

With
Christmas approaching, I began to think about buying a
transceiver kit. Of course, I could have started a new transceiver design and build project from
scratch, but I thought I'd take the easier route of building a kit, and
then use it as a 'test bed' for some other ideas.
Looking
around at the available options, I settled on the Lydford transceiver
kit from Walford Electronics. It can be built for any of a number of HF
bands, but I decided I would built it for use on the 40m band.

The parts in the kit will allow you to duplicate the kit as
pictured on
the Walford website. That's the photo you can see below. i.e. The parts
supplied in the kit provide the basics only, including a simple
PCB for use as the front
panel and for mounting the main tuning capacitor (on the left), the
fine
tuning control (centre) and the AF gain (volume) control (right) as
well as sockets for the microphone and headphones or external speaker.
Obviously, I've added some extra stuff for my version.

Figure 1 : The Lydford transceiver as pictured on the Watford website

I've mounted my Lydford transceiver board in an aluminium enclosure
obtained from Maplin in the UK, added a DDS VFO which I designed
specifically for the transceiver (and described elsewhere on this website here), added my own CW active filter and AGC circuitry, and some front panel artwork to make it all look pretty.

Figure 2 : My Lydford transceiver in its enclosure with digital VFO and all ready to go

But before we get to that, here are...

Some Initial Comments about the Kit

I like
this QRP transceiver, I really do. It has some interesting
design features, and it really does perform up to expectations. As a
kit, however, it is not the easiest to put together. It was clearly
identified as a more complex kit on Tim's website - It's found in the
'Advanced' section. Mind you, I
managed to built
it so it wasn't too difficult, and the transceiver works really well.

And it was not an
expensive
kit . It came well packaged along with a set of detailed
instructions running to 22 pages. These instructions describe the basic
circuit
operation and the kit assembly process, and some notes on
possible ways to extend the basic transceiver.

The kit instructions wee
complete, but they required much more effort to follow than many other
kits
where each step and the location of every component is carefully
described.
All of those details were provided but in a more
concentrated
manner, and it was in the "Advanced" section of their website where builders should have relatively good skills and equipment.

The schematic and layout diagrams were hand-drawn, but they contained
almost all of the details required. They were not
perhaps as clear as schematics and diagrams you might see in a typical
electronics magazine or, say, like those on my website. (Sorry - I couldn't resist the self-promotion there)

Most importantly, Walford's printed circuit boards (PCB) are laid out by
hand, just like we used to do them back in the 1970s. That means the PCB
lacks features you might reasonably expect in a modern kit, such as
perfectly aligned and well-spaced
components, a printed PCB overlay on the topside of the PCB to help in component placement, and a
solder mask on the copper side of the PCB. All of these make the
kit-builder's job a little easier. But Walford kits are not like that. So builder skills are definitely required.

Building my Lydford Transceiver Kit

Here
are a set of photos with some notes which describe the various stages
of my transceiver’s construction.

Being more difficult to build than some other kits, I began by reading the
instructions several times. I found I noticed
some extra details each time I went through them. I also used a highlighting
pen to mark where band-specific components had to be fitted to reduce
the chance of making mistakes.

I then carefully inspecting the PCB. Because these boards
are handmade, and not produced in a PCB factory somewhere in a distant
foreign land, you do have to carefully check the PCB for any errors.
It’s
best to look for these right at the start. They are usually easiest to
fix at this stage, too.

In my case, I noticed that a number of the drilled holes were not
centred, and several tracks looked doubtful. You can see some examples
in the photos below. In most
cases, they presented no problem. Occasionally, and I’ll describe an
example from my PCB, you may need to drill another hole or two.
Where I noticed some tracks that looked doubtful (See Figure 4 below -
Right-click on the photo with your mouse if you want to see the full detail),
I dropped a blob of solder on each location on the tracks where they
looked they might be open-circuit. Checking with an ohm-meter, they
appeared to be OK, but I added the solder blobs just to be sure.

I then checked all the parts against the parts list, as suggested in the
kit instructions. I had one of the first or second batches of Lydford
kits sold and there were a few minor differences between the
parts list, the schematic and what was supplied in the kit. Nothing
serious, and Tim has since updated the parts list so new builders
should not encounter any problems.
I usually also make a photocopy of the circuit diagram and the layout
diagram and place them on my workbench right next to where I am
building a PCB. This allows me to glance from one to the other quickly
as I'm building the board. I also used my highlighting pen to mark the
components on the layout diagram as I fitted them on the PCB. This
helped me to more quickly identify the location for subsequent parts in
the absence of the printed component PCB overlay.

In this case, I found the circuit diagram was more difficult to work
from that I would have liked. It flows across two pages and, in some
areas, especially the switching around IC4 and IC6, the circuit's
operation was something of a mystery. Since I encountered a problem
later in this area which I had to fix, I ended up spending a few hours
early in the project redrawing the schematic onto a single page
using one of my PC-based design programs. I printed out onto an A4 page and then photocopying it up
to A3 size. No problems with reading it after that!

Figure
5 : Here is a section of the Lydford schematic which I redrew on my PC.
When I photocopied up to A3 size, it made making the kit much easier to build, track down faults, and (later) modify. The
copyright over the Lydford's circuit design is held by Walford Electronics. My redrawn schematic is not available for
download. Walford has refused permission to publish it even though the radio is no longer in production. And, no, sorry, I can't email it to you either.

Here’s how the board looked after the first steps. The power/antenna
connector is in place, the front end coils installed, the trimmer
capacitors, along with a few miscellaneous parts.

A couple of new parts (IC2, C20, C21) have now been mounted on the PCB,
but things still look pretty lonely on this part of the estate. You can
clearly see the countersunk method to isolate the top ground plane in
this PCB. Unlike modern PCBs, there is a remote chance that approach can result in a short circuit
should an angled component wire touch the ground plane at the edge of
this hole. It pays to keep a close eye out for this potential problem
as you build the kit, especially with the angled leads of the transistors.

Continuing to follow the assembly instructions, I then completed and tested the receiver’s speaker amplifier.

The audio preamp parts around op-amp IC7 were then added, and a
speaker and (at the lower left corner in the photo below) the volume pot were temporarily wired
in place for testing it, and subsequent stages. I used a 30mm
diameter 2mm thin speaker for this. I replaced it later with a more sensitive full-sized speaker but this tiny speakeris a useful size for testing during the build phase.

Bt the way, I much prefer this "stage by stage" approach to kit
building instructions compared to the alternative used in some kits from other suppliers.
Each stage can be tested, and the kit gradually comes alive, section by
section. In some other kits, the builder is led through adding sets of
the same value component, for example a bunch of 1k resistors all over the board, then a set of 1k5
resistor, and so forth, until every component is fitted. Then the
equipment is powered up and tested. For me, that approach is quite
boring. It can be problematic too. For example, faults can be much more
difficult to locate. The Walford approach, stage by stage assembly and
testing, is much more interesting and satisfying.

I proceeded next to install the second mixer (IC5) and switching (IC6). It was here that I then encountered some problems with the voltage measurements described in the instructions.After a
few minutes work, I found a fault in the PCB layout which required a
little bit of careful work with a scalpel to fix. You won’t encounter
this problem – Tim’s fixed the artwork to better ensure your
transceiver will work for you first time. In this case, you may just be
able to see in the photo below where I’ve removed some unwanted tracks
which lay between several of the IC pads.

Having sorted that problem out, it was time to install the crystals for
the IF filter. This filter provides most of the selectivity in the
receiver. Here I found another little problem – If you look carefully
at the top corner of the IC in the photo below, you’ll see the crystal
case which I placed here temporarily is grounding pin 1 and pin 2 of
IC6. I re-drilled the mounting holes for this crystal, offsetting them
by about 1mm, and all was well.

The crystal then fitted perfectly, with plenty of space separating it from IC6.

In the photo below, you can see that I've fitted all of the crystals
for the IF filter and the second switching chip (IC4) has also been
added. The real estate on the PCB is starting to look much busier.

The first mixer (IC3) and buffer transistor (TR5) were then added.
Nothing more to test just at the moment. To do that, we first need a
VFO.
But first, here’s a sideways view of the centre of the board across the
first mixer to the crystals making up the IF filter. Very “New York
skyline” appearance with those crystals in the background, perhaps. OK,
so maybe I've been breathing too many solder flux fumes.

As the next photo (below) shows, I then built the first oscillator
section of the VFO. L1, a T50-2 toroid with a bunch of turns on it, is
very much the feature of this
photo. My VFO is, of course, for the 40m version of the Lydford
transceiver, so
ideally it should tune from about 4.2 to 4.8MHz. The coil winding
instructions resulted in the VFO starting close to mid-range, around
4.4MHz. However, a few more minutes of work following the kit
instructions
had it tuning nicely across this range. Adding the (tiny!) SMD varicap
diode
on the underside of the PCB added fine tuning with the aid of another
potentiometer.

I quickly became
aware of the good stability of the VFO even though everything was still
very much out in the open air of my workbench. You can just make out the tuning
capacitor in the photo (above) peeping out from under the board. It was
just tack-soldered in place. Certainly, the VFO was only operating on 4MHz, but
it was still a very good performer compared to many oscillators around that frequency that I've had on my
workbench.The
main tuning capacitor in an analog VFO like this would ideally be tuned
using a reduction drive. An example of a reduction drive is shown on
the
right. This is a modest 6:1 unit, but for my transceiver, I needed a
more
elaborate drive. It needed to reduce the tuning rate from 150 – 200kHz
per
half-turn on the "polyvaricon" variable capacitor supplied in the kit
to a more reasonable turning rate of, say, 10 to 20kHz per turn of the
tuning knob. The Lydford kit
does come supplied with an additional ‘fine tuning’ varicap control for
the VFO to avoid the need for this sort of reduction drive. However,
this requires you to use a two-step tuning process, first tuning to the
general area of the band with the main control, and then fine tuning
onto the exact frequency with the second control. This works, but I
felt the usefulness of the transceiver would be improved if I used an
alternate approach.Lacking
a suitable reduction drive, or a vernier dial with its reduction
mechanism (An example of a vernier dial is shown on the right), and not
having access to a full-scale mechanical workshop and suitable parts to
make something better (A friction drive or a string-dial are two traditional
approaches that are possible), I had
planned from the outset to replace this section of the transceiver with
a DDS-based VFO once everything in the original kit was working.

A DDS VFO uses a rotary encoder for tuning (You can see an example of a rotary encoder here)
with the tuning speed set in software. DDS software will often support
several tuning speeds, and even variable rate tuning. The DDS VFO I
used is one of my own design. It has four selectable tuning rates (I don't like
the alternate 'variable rate tuning' approach) and it is described in more detail here on my website.

In the meantime, and as the next photo (below) shows, I continued using
the analog VFO during initial construction. The VFO signal is divided
by four in the 40m version of this transceiver. This uses a 74HC74 divider
which I then fitted on the PCB. It's in the lower-centre of the photo below.
The pink jumper to the right of the divider selects the divide-by-4
output required for this VFO, as well as for my DDS VFO that I added later.

The complete receiver was operating at this stage, and I connected it
to a 20m long-wire antenna. This immediately brought in a multitude of
signals, demonstrating the Lydford’s sensitivity. (Very happy smiles on
the face of the builder, and a gentle complaint from my ever-patient
wife in the next room about the loud CW and SSB signals audible from
the workbench. Headphones...Where are my headphones...?)

After some enjoyable time spent playing with the receiver, it was on to the transmitter stages, beginning with the
microphone preamplifier. These parts, including the microphone gain
control can be seen fitted in the photo below. The 8-pin NE612 second mixer can
also be seen in the lower left of this photo.

I didn’t have a suitable low impedance dynamic microphone which turned
out to be required with this microphone preamp. It wasn't mentioned on
the Walford website. With no
easy way to obtain one, I had to look for an alternative.

Electret microphones
are incredibly commonplace these days, with almost every transceiver,
recorder, wireless device and telephone using them. I had three or four
electrets microphone capsules in my parts bin, as well as some
ready-to-use electret microphones. Several of these were tried after I made a small
circuit modification to the Lydford's microphone preamp stage. This
modification can be seen in the schematic diagram below.

Figure 6 : Schematic showing the input section of the original
dynamic microphone preamp and the modification I made to use a more
widely available electret microphone

The
four additional components required were added under the PCB. They are
small enough to fit under the board without problem when the PCB is
mounted in place on the chassis. I then tested a number of different electret microphones. I'll describe that shortly.

Figure 7 : The added components for the electret microphone. The 10k
resistor at left is wired to the +8V rail, and the yellow wire trails
away to the electret microphone. A 100n disc ceramic capacitor provides
RF bypassing to an adjacent ground rail.

Here’s a close-up view of the microphone preamp and divider with a line
of four diodes heading in a line in the background. These drop the 8V
rail to 5V for the divider – A simple and effective solution that
avoids the use of another regulator and several bypass capacitors.

Meanwhile, the relays and a pair of presets, one to set the transmitter
drive level and the other to set the transmitter’s power amplifier bias
voltage, were added to the PCB. The relays can be seen at top-left and
top-right while these two presets can be seen to the right of the
crystal filter.

As you can see, the board is starting to look quite busy, yet at no
stage did it feel overwhelming. If
you saw this photo at the start, it might put you off, but actually
building this, little by little, is quite straight-forward. The most
important thing to remember is to check the location of each part VERY
carefully against the layout diagram in the instructions. Install the
part, then check its location again. Only then should you solder it in
place.

About
this stage of the kit, one of the parts which has to be added is D70.
This 1N4148 diode is fitted between IC6, the one of the CD4066
switching chips, and two of the crystals in the IF filter, X5 and X6. I
added some (blue) insulating sleeving to the leads on this diode as a
precaution against possible grounding to one or other crystals.
The next step was to build the next transmitter amplifier stages. The
photo below shows the first stage, a single FET, which you can see
tightly tucked in beside the relay to the centre-left of the photo
below. This grounded gate FET stage is DC-switched and, in transmit mode,
it buffers the low-impedance transmit output from the RF filters (L2,
L3 etc) produced by the mixer (IC3) and switching (IC4) through to the
high impedance input of the next stage (IC100 - AD8055) of the
transmitter.

The
subsequent stage, IC100 (AD8055) and a pair of transistors (TR101,
102),
was then installed. You might just be able to see a very thin -looking
100k resistor (R109A)
fitted between IC100 and the Drive preset in the top-centre of the
photo below. The 1/4W resistor supplied in the kit was a fairly tight
fit so I replaced it with
a smaller 1/8W type to maintain clearances here. This new resistor is operating well within its ratings, so no problems are expected.

Incidentally, I
labeled the RF Drive preset on the board at this stage because of the
identical appearance of these two adjacent presets. The other preset adjusts the
PA bias current. This was to prevent any mistakes later during alignment.

I've
not used the AD8055 before. It's a variable gain low noise amplifier
with gain adjustable in the Lydford from unity to more than 10dB. Its
frequency response at that gain setting extends beyond 15m (21MHz).

I tested my (40m) transmitter RF output from this stage into a 1k 1/4W
load temporarily soldered on the underside of the PCB where the gate
and source of TR100 (IRF510) would normally be installed. Everything
worked perfectly, with speech from the electret microphone producing
1.2Vpp across this temporary load resistor. That matched the value
suggested in the instructions.

As I mentioned earlier, I tried several electret microphone capsules as
well as the electret microphone fitted in a cheap computer headset, and
I even tried an
external microphone fitted in an iPhone accessory microphone/earphone.
The
output varied considerably between all of these different microphones.
The cheap computer headset was the least sensitive, while a small
microphone capsule measuring about 4mm in diameter from an old cordless
phone produced the most output. Ideally, I would have redesigned and
rebuilt the
entire microphone preamp stage to better suit these common-as-dirt
electret microphones, but I was trying to leave as much as possible
unmodified at this stage. (Some months later, I tried a "dynamic
microphone" purchased in the UK. That's what the label said. Well, it
turned out to be an electret type!)

The PA and its towering heatsink were fitted next. I had my doubts
about the need for such a large heatsink, but these thoughts were quickly
banished. The heatsink does get warm during use, but not particularly
hot. It also provided a significant margin of safety, protecting the PA FET
from any possible harm while I aligned the transmitter. That was especially true
during my lengthy tests of a variety of microphones and accessories. (This photo
below makes the heatsink look like the Tower of Babel, but it's actually only
about 30mm high!)

The last step in the PCB construction was winding and fitting the three
output toroids, two of which are used in the output filter. The
selection and fitting of the required capacitors took a little more
time, especially the time taken to carefully work out which PCB holes to use. It's
fairly crowded in that section of the board.

Actually, I temporarily removed C109 (100uF) from the PCB to give me a
little more space in which to work. After fitting the low pass filter
components, I then refitted C109 before testing the transmitter again.

As
I've noted, my transceiver was built for use on 40m, so if your
transceiver is
built for another band, your toroids will have a
different number of turns on them and require different capacitors, so
things may look a little different here compared to my PCB.

The last test of my newly completed transmitter (into a 50 ohm dummy load) worked exactly as expected.
There were no signs of instability or strange spurious outputs, and I
managed to achieve about 5W out with a power supply voltage of just
under 13V.

Final alignment took place later once the PCB was mounted in the chassis and wiring was completed.

Finishing Construction

Here is
the transceiver board fitted into its chassis, with the wiring nearing
completion. The plain unfinished aluminium chassis was purchased from Maplin UK.

The prototyping board which can be seen mounted on the front panel is
my DDS VFO. Developed specifically for this radio, its display is fitted to the front panel. The main
Lydford transceiver board can be seen mounted on the right of the
chassis with wiring going to the antenna and DC connectors on the rear panel and to the front panel controls.

Don't be misled by the current showing on my power supply in the
background. The Lydford's receiver only requires a modest 50mA. The remainder
of the current is mostly drawn by the very hungry AD9850 DDS chip used in the DDS VFO.

Some Accessories for my Lydford QRP Transceiver

The
other (brown) prototype board seen hanging out from the left hand side of
the transceiver
in the photo above is an AGC circuit being tested. The Lydford receiver
is quite sensitive, particularly with the low noise floor of the analog
VFO. As I tested the receiver and tuned across the 40m band, I found
listening to the vast range of different audio output levels, some
signals being ear-numbingly loud while others were relatively quiet,
rapidly became tiring. This is due to the lack of AGC in the Lydford
transceiver. If I was using this transceiver back home in New Zealand,
the number of broadcast stations and the audio level variations I
encounter when
tuning might not be quite so great. But here in the Middle East, I've
got large and small signals right on my doorstep. So, adding AGC
quickly became essential.

To be fair, I was aware of the potential need for AGC before I started
the kit, but I wanted to see for myself just how necessary it was in
this transceiver and in my location. With a few megawatts of MW and SW
transmitter power less than 30 minutes drive away from my house, the need
for AGC quickly became obvious!

I did not have the forethought to buy the matching Walford AGC kit, and
the lack of a reliable mail service prevented it being sent to me
here in the Middle East. So, as usual, I ended up experimenting with a number of
alternative circuits and approaches. The one pictured in the photo above was based on a design used
in the BitX and IFER SSB QRP transceivers. It was later replaced with
another design which produced considerably less noise. The resulting combined
AGC and CW filter board is described elsewhere on my website. The basic CW filter I added to this board also made listening to the CW signals located between the broadcasters at the lower edge of the 40m band much more enjoyable.

I also
built some other accessories for the transceiver, and most
are described here on my website. These include a
“silent” antenna tuner, so no RF output is required from the transmitter
until the very last stage of tuning the ATU (shown on top of the finished transceiver in the photo above), and an RF power
meter which is mounted inside the ATU. I may also add
an S-meter to the transceiver at some future date. For more details of these added items, just click on the links below.